Control circuit of power supply unit which controls output power of external power supply based upon current from the external power supply, power supply unit and control method thereof

1. A control circuit of power supply unit for controlling output voltage with electric power supplied from an external power supply, comprising: a monitoring portion monitoring current and voltage outputted from the external power supply and the output power of the external power supply; and a setting portion setting the upper limit value of current outputted from the external power supply based on a result of monitoring by the monitoring portion, wherein the monitoring portion includes: a voltage detecting portion detecting that voltage value outputted from the external power supply is smaller than a lower limit value; a current detecting portion detecting that current value outputted from the external power supply is limited by the upper limit value; an upper limit value changing portion changing the upper limit value set by the setting portion based on a detection result of the voltage detecting portion or the current detecting portion; an electric power calculating portion calculating a first electric power from current value and voltage value outputted from the external power supply based on the upper limit value set by the setting portion or a second electric power from current value and voltage value outputted from the external power supply based on the upper limit value changed by the upper limit value changing portion; an electric power comparing portion determining which is larger the first electric power or the second electric power calculated by the electric power calculating portion; and an adjusting portion, in accordance with a result of determination by the electric power comparing portion, replacing the upper limit value changed by the upper limit value changing portion with an adjustment upper limit value different from the upper limit value.

2. The control circuit of power supply unit according to claim 1 wherein when the voltage detecting portion detects that the voltage value outputted from the external power supply is smaller than the lower limit value, the upper limit value changing portion lowers the upper limit value set by the setting portion.

3. The control circuit of power supply unit according to claim 1 wherein when the current detecting portion detects that the current value outputted from the external power supply is limited by the upper limit value, the upper limit value changing portion raises the upper limit value set by the setting portion.

4. The control circuit of power supply unit according to claim 1 wherein under a condition that the electric power comparing portion determines that the first electric power is larger than the second electric power, the adjusting portion replaces the adjustment upper limit value with an upper limit value set by the setting portion at the time of calculating the first electric power.

5. The control circuit of power supply unit according to claim 1 wherein when the current detecting portion detects that current value outputted from the external power supply is limited by the upper limit value after the upper limit value changing portion raises the upper limit value set by the setting portion, under a condition that the electric power comparing portion determines that the first electric power is larger than the second electric power, the adjusting portion replaces the adjustment upper limit value with an upper limit value set by the setting portion at the time of calculating the first electric power.

6. The control circuit of power supply unit according to claim 1 wherein, under the condition that, after the upper limit value changing portion raises the upper limit value set by the setting portion, the voltage detecting portion detects that voltage value outputted from the external power supply is smaller than the lower limit value, and the electric power comparing portion determines that the first electric power is larger than the second electric power, the adjusting portion replaces the adjustment upper limit value with an intermediate value between upper limit value set by the setting portion at the time of calculating the first electric power and upper limit value set by the setting portion at the time of calculating the second electric power.

7. A power supply unit controlling output voltage with electric power supplied from an external power supply, comprising: a monitoring portion monitoring current and voltage outputted from the external power supply and the output power of the external power supply; and a setting portion setting the upper limit value of current outputted from the external power supply based on a result of monitoring by the monitoring portion, wherein the monitoring portion includes: a voltage detecting portion detecting that voltage value outputted from the external power supply is smaller than a lower limit value; a current detecting portion detecting that current value outputted from the external power supply is limited by the upper limit value; an upper limit value changing portion changing the upper limit value set by the setting portion based on a detection result of the voltage detecting portion or the current detecting portion; an electric power calculating portion calculating a first electric power from current value and voltage value outputted from the external power supply based on the upper value set by the setting portion or a second electric power from current value and voltage value outputted from the external power supply based on the upper limit value changed by the upper limit value changing portion; an electric power comparing portion determining which is larger the first electric power or the second electric power calculated by the electric power calculating portion; and an adjusting portion, in accordance with a result of determination by the electric power comparing portion, replacing the upper limit value changed by the upper limit value changing portion with an adjustment upper limit value different from the upper limit value.

8. The power supply unit according to claim 7 wherein when the voltage detecting portion detects that the voltage value outputted from the external power supply is smaller than the lower limit value, the upper limit value changing portion lowers the upper limit value set by the setting portion.

9. The power supply unit according to claim 7 wherein when the current detecting portion detects that the current value outputted from the external power supply is limited by the upper limit value, the upper limit value changing portion raises the upper limit value set by the setting portion.

10. The power supply unit according to claim 7 wherein under a condition that the electric power comparing portion determines that the first electric power is larger than the second electric power, the adjusting portion replaces the adjustment upper limit value with an upper limit value set by the setting portion at the time of calculating the first electric power.

11. The power supply unit according to claim 7 wherein when the current detecting portion detects that current value outputted from the external power supply is limited by the upper limit value after the upper limit value changing portion raises the upper limit value set by the setting portion, under a condition that the electric power comparing portion determines that the first electric power is larger than the second electric power, the adjusting portion replaces the adjustment upper limit value with an upper limit value set by the setting portion at the time of calculating the first electric power.

12. The power supply unit according to claim 7 wherein, under the condition that, after the upper limit value changing portion raises the upper limit value set by the setting portion, the voltage detecting portion detects that voltage value outputted from the external power supply is smaller than the lower limit value, and the electric power comparing portion determines that the first electric power is larger than the second electric power, the adjusting portion replaces the adjustment upper limit value with an intermediate value between the upper limit value set by the setting portion at the time of calculating the first electric power and the upper limit value set by the setting portion at the time of calculating the second electric power.

13. A control method of power supply unit controlling output voltage with electric power supplied from an external power supply, comprising: monitoring current and voltage outputted from the external power supply and the output power of the external power supply; and setting the upper limit value of current outputted from the external power supply based on a result of the monitoring, wherein the monitoring comprises: detecting that voltage value outputted from the external power supply is smaller than a lower limit value; detecting that current value outputted from the external power supply is limited by the upper limit value; changing the upper limit value set by the setting the upper limit value of current based on a detection result of the detecting voltage value or a detection result of the detecting current value; calculating a first electric power from current value and voltage value outputted from the external power supply based on the upper limit value set by the setting the upper limit value of current or a second electric power from current value and voltage value outputted from the external power supply based on the upper limit value changed by the changing; determining which is larger the first electric power or the second electric power calculated by the calculating; and replacing the upper limit value changed by the changing with an adjustment upper limit value different from the upper limit value, in accordance with a result of determination by the determining which is larger the first electric power or the second electric power.

14. The control method of power supply unit according to claim 13 wherein when the detecting voltage value detects that the voltage value outputted from the external power supply is smaller than the lower limit value, the changing lowers the upper limit value set by setting the upper limit value of current.

15. The control method of power supply unit according to claim 13 wherein when the detecting current value detects that the current value outputted from the external power supply is limited by the upper limit value, the changing raises the upper limit value set by the setting the upper limit value of current.